Many types of equipment are designed to detect physical events such as particle-matter interactions. Event detection is widely used in scientific research and in medicine. An example of useful event detection equipment is a nuclear medicine camera, also referred to as a gamma camera. Such cameras can aid in locating diseased tissue, such as tumors, in the body.
Some conventional nuclear medicine imaging systems have two or more detectors. The detectors are usually planar and include an array of detector devices such as photo multiplier tubes (PMTs). The detectors arrays are positioned above different sides of a patient. Gamma cameras can operate in different modes. For example, some nuclear medicine cameras perform single photon emission computed tomography (SPECT) in which information from a single detector is used to produce information. Other nuclear medicine cameras perform positron emission tomography (PET) in which the detection of two scintillation events, one in each of two detectors that occur 180.degree. apart, are used to compute imaging information. In a PET system, detectors detect scintillation events that result when a photon of a photon pair collides with a crystal.
Before the gamma camera is used in PET mode, the patient is injected with a radiopharmaceutical, such as Flouro Deoxi Glucose (FDG). The radiopharmaceutical emits positrons that interact with electrons in the body. As a result of the interaction, the positrons are annihilated and gamma rays, including photon pairs, result. Photon pairs leave the scene of the interaction in directions of travel that are 180.degree. apart from each other. When a photon comes in contact with a crystal of a detector, a scintillation event occurs. The scintillation event is detected by a photo detector device of the detector creating analog information. The analog information is digitized and processed by electronics and software to produce image information about objects such as tumors in the body.
In SPECT mode, the patient is injected with a radiopharmaceutical and emissions from the patient are then detected independently by detectors of the gamma camera, rather than detecting coincident events. When a trigger signal from a detector is received by processing hardware and software, the outputs of the PMTs are integrated to determine the energy associated with the event. Integration occurs during a constant time window. Therefore, if different trigger signals line up differently with respect to the timing window, some events will be processed to show energy in one area of a spectrum, and another event in another area of the spectrum. All energy spectra are added to generate a global energy spectrum for the system. The narrower the energy distribution, the more accurate the spectrum will be. Ideally, energy resolution should depend primarily on the spectra of the crystal and on the PMT energy. Some inaccuracies are unavoidable, such as variance in process behavior in the crystal by location, and noise in the PMT that prevents signals captured from each gamma ray from being the same. Variations in trigger signal arrival time add to inherent inaccuracies of the SPECT method because they affect energy resolution.
Another example of event detection equipment is a Compton camera. A Compton camera examines coincident events between two detectors to determine the well-known Compton scattering angle associated with an interaction of photons with matter. Known equations are used to find the Compton angle, which in turn is used to calculate the position of the event in the body.
Typical gamma camera, regardless of the mode in which they are operated, include detectors with multiple devices such as PMTs. For various reasons, the propagation time of trigger signals indicating detection of events varies between PMTs. One factor contributing to propagation time variance is the fact that PMTs vary physically in ways that affect their response times. Another factor is the variance in the length of cables used to carry signals associated with different PMTs. Yet another factor is crystal response time variance by area. If the trigger signal is received by processing hardware and software significantly later than the event detected, inaccuracies may result. Inaccuracies may include false detection indications, and images with poor resolution. Therefore, it is critical to calibrate the timing of trigger signals so that they portray, as accurately as possible, what is actually occurring in the tissue of the patient.
Proper calibration of trigger signals can be important in both PET and SPECT systems, as well as systems that include Compton cameras. Currently, calibration is performed on PET systems by positioning a radioactive source between detectors and monitoring rates of coincident events. Such methods of calibration are often time consuming and may be imprecise because the steps performed are not accurately repeatable. Currently, no calibration is known to be performed on detectors of systems, such as SPECT systems, that do not operate by detecting coincident events.
Detector calibration is especially critical in systems, such as PET systems and systems using Compton cameras, that rely on detection of coincident events to produce imaging data. If the collision of one photon of a photon pair with one detector is not reported at the same time as the collision of the other photon of the photon pair with another detector, the coincident event will be missed. Techniques currently exist for calibrating PET systems, but these techniques have several disadvantages. Current techniques are complex and not accurately repeatable. In addition, current calibration operations take a relatively long time to perform.
FIG. 1 is a block diagram of a prior PET coincidence detection system 200. System 200 includes two detectors 266 and 268. Detector 266 is divided into four zones 266 (1), 266 (2), 266 (3), and 266 (4). Each of zones 266 (1-4) include multiple PMTs. Zone 266 (1) and its connected components operate similarly to other zones in detector 266 and corresponding zones in detector 268. Zone 266 (1) will be described as an example of how a zone of a detector operates. When any PMT in zone 266 (1) detects a scintillation event resulting from a collision of a photon with the crystal of detector 266 (not shown), an analog signal is sent to summing circuit 201. Summing circuit 201 receives signals from all of the PMTs in zone 266 (1) and sums their amplitudes in a known manner. Summing circuit 201 outputs a signal to constant fraction discriminator (CFD) 221. CFD 221 operates as a trigger detector in an amplitude independent manner. CFD 221 outputs a zone trigger signal to programmable delay 241. Programmable delay 241 is typically controlled by a processor of system 200 and is used to vary the delay of the trigger signal output by CFD 221 during calibration of system 200. Zones 266 (2), 266 (3), and 266 (4) operate in the same manner as zone 266(1), each outputting a signal from their respective programmable delay circuits indicating that an event has been detected. The outputs of programmable delays 241, 242, 243, and 244 are input to OR gate 256. Detector trigger signal 270 is active on the output of OR gate 256 when any event is detected in a zone of detector 266. Detector trigger signal 270 is input to common delay CD.sub.A 260, which is associated with a detector A. CD.sub.A 260 is a programmable delay circuit that is used to vary the delay of detector trigger signal 270 with respect to detector trigger signal 272. Adjusted detector trigger signals 274 and 276 are input to coincidence detection circuit 264. Coincidence detection circuit 264 typically performs an operation such as an AND operation for determining when scintillation events have been detected simultaneously in detectors 266 and 268.
Programmable delays 241, 242, 243, and 244 are adjusted to compensate for variances such as response time of different PMTs, different cable lengths between zones, and different crystal response times between zones. In a typical prior art method, which will be described below, delays 241-244 are adjusted, as well as delays 251-254 associated with detector 268. Then, common delays CD.sub.A 260 and CDB 262 are adjusted for the purpose of synchronizing the global, or common, delays of signals 270 and 272 with respect to each other. Programmable common delay circuits CD.sub.A 260 and CDB 262 adjust for delays due to cable lengths between OR gates 256 and 258 and coincidence detection circuit 264.
FIG. 2 is a flow diagram showing steps of a prior art calibration operation that may be applied to a system such as system 200. At steps 302, the delay of a single zone X of one of the detectors, called detector A, is set to a nominal value. The nominal value is a combination of an individual delay of the single zone and a common delay between the two detectors. The nominal value of the delay of zone X is typically determined by calculating known cable delays and component response delays in the path of zone X.
Zone X is used as a reference zone to set delays of detector B zones. At step 304, a point source is positioned between zone X and a zone of detector B. Then, at 306, the delay of the zone of detector B is adjusted until a maximum coincidence rate is sensed. As the point source radiates, causing coincident events, a processor receiving data from detectors A and detector B calculates the rate at which coincident events occur. When the maximum rate is sensed, it is assumed that the trigger signal generated by detector A and detector B have close to the same flight times. Steps 304 and 306 are then repeated for each of the four zones in detector B, as shown at 308.
At 310, a zone of detector B is chosen as a reference zone Y. A point source is then positioned at 312 between zone Y of detector B and a zone of detector A to be calibrated. At 314, the delay of the zone of detector A is adjusted until a maximum coincidence rate is sensed. Steps 312 and 314 are repeated until the our zones of detector A are calibrated, as shown at 316. Finally, the individual zone delays and common delays are set for each of the two detectors at 318 to achieve the smallest amount of common delay.
The disadvantage of the method shown in FIG. 2 is the necessity of adjusting the position of the point source between detectors A and B for each zone to be calibrated. A disadvantage is that the exact positions of the point source with respect to detectors 266 and 268 are not repeatable with a high level of accuracy. This makes diagnosis of system behavior difficult and inaccurate. Another disadvantage is the length of time the calibration process takes. The detectors shown each include only four zones and the calibration operation typically requires approximately thirty minutes to complete. Detectors having more zones than four would be more efficient in detecting coincident events, but calibration time of such detectors with the prior art method of FIG. 2 would increase as the number of zones increased. For example, the number of measurements required by the method of FIG. 2 is: EQU (number of zones in detector A)+(number of zones in detector B)-1=7
If each detector had 14 zones, 27 measurements would then be required by the method of FIG. 2.